Oligosaccharides are chains composed of saccharide units, which are commonly known as sugars. Of the biological polymer families, oligosaccharides are the least studied, due in part to the difficulty of sequencing and synthesizing their complex sugar chain. Currently, no generally applicable synthetic technique for synthesizing oligosaccharides is available.
Intensive research efforts have been devoted to carbohydrates and molecules comprising carbohydrate fragments, such as glycolipids and glycoproteins. Research interest in these moieties has been largely due to the recognition that interaction between proteins and carbohydrates are involved in a wide array of biological recognition events, including fertilization, molecular targeting, intracellular recognition, and viral, bacterial, and fungal pathogenesis. It is now widely appreciated that the oligosaccharide portions of glycoproteins and glycolipids mediate cell-cell interactions, cell-ligand interactions, cell-extracellular matrix interactions, and cell-pathogen interactions.
It is thought that many of these interactions can be inhibited by oligosaccharides that have the same sugar sequence and stereochemistry found on the active portion of a glycoprotein or glycolipid involved in the interactions. The oligosaccharides are believed to compete with the glycoproteins and glycolipids for binding sites on the receptor proteins. For example, the disaccharide galactosyl-β(1,4)-N-acetylglucosamine is believed to be one component of the glycoprotein which interacts with receptors in the plasma membrane of liver cells. Thus, oligosaccharides and other saccharide compositions that mimic ligands recognized and bound by cellular receptors are thought to be useful in applications that include diagnostics and therapeutics.
In addition to mediating numerous cellular interactions, many oligosaccharides are recognized by the immune system. For example, Anti-Gal, a naturally occurring antibody present in all humans, specifically interacts with the carbohydrate epitope Gal-α(1-3)Gal-β(1-4)GlcNAc-R (α-galactosyl epitope). This antibody does not interact with any other known carbohydrate epitope produced by mammalian cells (Galili, Springer Seminar Immunopathology, 15:153 (1993)). Anti-Gal constitutes approximately 1% of circulating IgG (Galili et al., J. Exp. Med., 160:1519 (1984)) and is also found in the form of IgA and IgM (Davine et al., Kidney Int., 31:1132 (1987); Sandrin et al., Proc. Natl. Acad. Sci., 90:11391 (1993)). It is produced by 1% of circulating B-lymphocytes (Galili et al., Blood, 82:2485 (1993)). Accordingly, the ability of carbohydrates to elicit an immune response can be utilized to increase the effectiveness of vaccines against many types of pathogens by linking such a carbohydrate to a vacccine to increase the immune response to the vaccine.
There has been relatively little effort to test oligosaccharides as therapeutic agents for humans or animal diseases however, as methods to synthesize oligosaccharides have been unavailable as noted above. Limited types of small oligosaccharides can be custom-synthesized by organic chemical methods, but the cost of such compounds is typically prohibitively high. In addition, it is very difficult to synthesize oligosaccharides stereospecifically and the addition of some sugars, such as sialic acid and fucose, has not been effectively accomplished because of the extreme lability of their bonds. Improved, generally applicable methods for oligosaccharide synthesis are thereby desired for the production of large amounts of widely varying oligosaccharides for therapeutic purposes. Accordingly, the present invention provides enzymes and methods that can be used to promote the chemical linkage of numerous sugars that have previously been difficult to link.